The invention relates to the use of stress relieving slots in the forming of perimeter strip grid corner-pieces of increased flatness.
The grids are elements of nuclear fuel assemblies and particularly are useful as nuclear fuel assembly support grids designed to avoid hang-up of grid corners during insertion into the core of a nuclear reactor vessel. In the past, nuclear fuel grids were constructed with non-formed beveled corners. When loading or unloading nuclear reactors with individual fuel assemblies, assemblies diagonally adjacent to each other could become caught at the outer corner of the grid. This type of hooking has in the past lead to destruction of the perimeter strips of grids, so that the assemblies could not be re-inserted into the core. This is especially true for fuel assemblies which have been bowed during operation.
To overcome this problem, it has been proposed to use formed corners. See U.S. Pat. No. 4,705,663 to J. Steven. The corner would be formed by pushing the strip material inward toward the corner fuel rod at the top and bottom strip edges, thus causing the corner to curve inward forming a beveled corner and providing varying radii in the transverse direction with its outer longitudinal portions having a greater radius than its inner portion.
Since the corner has a larger bevel the problem of grid hooking less likely. However, these corners have not been used in grids with small cut-out features in the perimeter strips because flexing deformation occurred which destroyed the flatness of the flat side sections on either side of the transverse corner bend line. These deformations were caused by corner forming. This occurrence was denoted "oil canning" and was caused by attempts to design "camming-corner" perimeter strips.